Staged catalytic cracking reaction systems have been introduced to improve the overall octane quality of gasoline. In recent times, however, octane problems have been minimized and environmental constraints have had a larger impact on the refiner. As a result, the known staged catalytic cracking processes are not sufficiently effective in concomitantly meeting environmental constraints and maintaining a high quality octane gasoline product.
U.S. Pat. No. 5,152,883 discloses a fluid catalytic cracking unit which includes two catalytic cracking reaction steps in series. After hydrocarbon feed is cracked in a first catalytic cracking reaction step, light hydrocarbon gases and gasoline products are removed from the product stream and the heavier product portion is hydrotreated. Following hydrotreating and further gasoline product removal, the heavier hydrotreated product is cracked in a second catalytic cracking step. The gasoline products are removed and the heavier products are recycled into the hydrotreating process.
Rehbein et al., Paper 8 from Fifth World Petroleum Progress, Jun. 1-5, 1959, Fifth World Petroleum Congress, Inc., New York, pages 103-122 (which corresponds to U.S. Pat. No. 2,956,003, Marshall et al.), disclose a two stage catalytic cracking process which uses a short contact time riser as the first stage. The first stage is described as being designed to give 40-50 wt % conversion. The second stage is a dense bed system that is stated as being designed to charge gas oils from the first stage along with a recycle stream to give overall conversions of 63-72 wt %, although the unit is said to have been run at low enough charge rates to achieve total conversions from 65-90 wt %.
As the prior art demonstrates, known catalytic cracking processes which have been integrated with hydrotreating processes are effective in significantly increasing the octane level of the gasoline product. The known systems, however, increase octane by sacrificing the quality of distillates which can be used as diesel or heating oil. In addition, the known processes produce a relatively high quantity of light saturated vapor products as a result of undesirable hydrogen transfer of hydrogen from the heavier cracked products back to lighter olefin products. By minimizing the negative effects of this type of hydrogen transfer, a greater quantity of olefins product can be produced, and these olefins are made available for further conversion into oxygenates and useful polymer materials. It is, therefore, desirable to obtain a catalytic cracking process which maximizes olefins production, distillate quality and octane level.